Electronic component-carrying substrates (e.g., circuit boards) are commonly cleaned to remove solder flux residues that result from soldering components to the substrate. Traditionally, component-carrying substrates have been cleaned by solvent spraying or solvent immersion. While solvent cleaning effectively removes solder flux and other undesirable residues, the advent of stringent environmental regulations has made solvent cleaning more difficult and costly to implement. For that reason, much effort is being devoted to developing solvent-free cleaning techniques. One approach for accomplishing solvent-free cleaning of component-carrying circuit substrates has been to direct solid CO.sub.2 pellets at the substrate at a high velocity. Although the exact mechanism by which such pellets clean the substrate is not completely understood, it is believed that upon striking the substrate, the CO.sub.2 pellets act to abrade the residues and carry them from the substrate.
One disadvantage of cleaning a component-carrying substrate in this fashion is that the CO.sub.2 pellets strike the substrate and tribocharge (electrify by contact and separation) the substrate, including its insulative and conductive portions. In some instances, the resultant charge on the pellets and/or the substrate may be sufficient to damage or destroy the electronic components on the substrate. We have discovered that in order to effectively mitigate electrostatic discharges, a water film must be produced on the substrate surface. The film of water is needed so that the water can combine with the CO.sub.2 to produce carbonic acid (H.sub.2 CO.sub.3) that dissociates into free ions that neutralize the charge on the CO.sub.2 pellets and the substrate (including the insulative and conductive portions thereof). In practice, we have found that spraying a water mist on the substrate during solid CO.sub.2 cleaning greatly reduces the incidence of electrostatic discharges. However, the incidence of electrostatic discharges was not completely eliminated because the sprayed water mist did not uniformly adhere to the substrate but tended to form puddles instead.
Thus, there is a need for a technique for eliminating virtually all electrostatic discharges associated with cleaning an electronic-carrying substrate with solid CO.sub.2.